Certain elements in the periodic table of elements, and compounds of those elements, as well as compounds of other elements that separately do not phosphoresce, exhibit phosphorescence. These materials are notable inasmuch as they absorb electromagnetic energy across a variety of wavelengths, the spectrum of absorption of which being specific to the element/compound. This energy absorption causes some of the atoms within the mass of the material to become excited above their ground state. This atomic excitation is transitory. The excited atoms subsequently re-emit photons as they return to their ground state.
These new photons are emitted at wavelengths that are the same as or (more usually) different from and more uniform than those absorbed during excitation. The excited atoms return to the ground state by emitting electromagnetic energy (photons), conforming to the law of the conservation of energy, over a variable time interval after excitation. Thus, the excited state of the material has a half life. After a given period of time (which varies with the material) one half of the excited atoms will have emitted a characteristic photon in the process of returning to the ground state. This time interval can be extremely short (e.g., the phosphors used in modern color television picture tubes) or very long (e.g., a glow-in-the-dark toy) possibly lasting for an hour or more.
The phosphorescent materials may be combined to form thousands of compositions. The phosphorescent materials and compositions thereof provide thousands of formulations of materials which exhibit unique emission characteristics. In particular, the wavelength and half life emission characteristics of the phosphorescent materials and compositions vary. The unique emission characteristic provides a "signature" of the respective phosphorescent material or composition.